A heating and cooling system of the above type is disclosed in the German publication No. DE-OS 22 31 117. In this type heating and cooling system, after the engine is hot the air for heating the car interior is heated as it flows through the engine radiator. The heating device, which is mounted ahead of the radiator, is operated by the same fuel that is used for running the engine and not only serves as an auxiliary heater in that it heats the occupant compartment while the engine is still cold, but also expedites the warm up of the engine after starting, because the air, after being heated by the engine, is first flowing through the engine radiator, whereby some of its heat is transferred to the engine cooling system.
While the heating and cooling system disclosed in the German publication No. DE-OS 22 13 117 is capable of rapidly warming up the engine and heating the car interior, it requires a relatively large amount of energy and thereby raises the fuel consumption of a vehicle equipped with such a system to an undesirable degree.
In the German publication No. DE-OS 27 05 748, another type of heating and cooling system is disclosed which incorporates an exhaust gas heat exchanger for warming up the water of the engine cooling system. The system for heating the car interior includes a heat exchanger which is mounted in parallel with the engine radiator and which is adapted to raise the temperature of the air used for heating the occupant compartment. However, as long as the engine is still cold, the water of the engine cooling system is directed, by means of a thermostatic valve, exclusively through the engine radiator, so that the engine will warm up to its operating temperature in a relatively short period of time. With the vehicle being still at rest or moving very slowly, the radiator is relatively ineffective during this warm up period. As a result, the heating system for the occupant compartment is supplied with little heat during this engine warm up period tending to cause fogging of the windows at low ambient temperature. While an exhaust gas heat exchanger of this type is useful for shortening this operating phase in comparison to ordinary heating systems, the engine warm up period is still relatively long under low temperature weather conditions.